The use of breakwater systems and other wave damping devices for the protection of shore areas from the damage caused by energy in sea or ocean waves is well-known. Many attempts have been made to stop or decrease beach erosion by reducing wave energy. It is also known to provide protection to man-made structures such as bottom-mounted oil platforms and structural objects in harbors from the damage caused by ocean waves.
Waves are generated at sea or lakes by virtue of the frictional drag exerted on the water surface by winds. It has been determined that the waves consist of water predominantly in vertical motion with little transverse motion in the direction of wave propagation. The individual particles move in a vertical plane orthogonal to the direction of wave propagation in a circular or substantially elliptical path. The particle orbit decreases in amplitude with an increase in the distance of the particle from the water surface. Thus, a particle at the surface traces an orbit whose vertical amplitude equals the vertical distance between the crest and trough of a wave traversing that point while others removed a distance from the surface experience lesser excursions.
Various devices are known in the prior art which are designed to obtain some reduction of wave amplitude.
For example, U.S. Pat. No. 3,011,316 discloses a breakwater device including flapper plates oriented vertically and protruding above the water surface. This device is rigidly implanted in the bottom near a beach to reduce its erosion and primarily affects the horizontal component of the elliptical water particle motion and does not appear to provide effective damping of deep water waves.
U.S. Pat. No. 4,006,598 to Hulsemann, discloses another example of a breakwater system for protection of the shore area and damping of incoming wave action. The system is designed to act beneath the air-water interface at the base of incoming waves to reduce the height of the wave. The system includes a plate-like structure designed to essentially "raise" the bottom, causing the waves to break farther off the beach than they would otherwise. The water passes vertically though the network of plates and is impeded equally in both the upward and downward directions.
U.S. Pat. No. 4,130,994 suggests another device primarily intended for shallow water surf zones. This device includes a large number of vertically-oriented strings of buoyant discs moored to the bottom of the sea. The major disadvantage is that the likelihood of entanglement of adjacent strings is so great as to render the device inoperable in high seas. In addition, since the mass of water attained varies as the cube of the radius of the disc, the device is far less effective than a single larger plate provided to effect the amplitude of the waves.
U.S Pat. Nos. 3,986,471 and 3,218,809 disclose devices which include a floating or surface-following struture from which negatively buoyant braking elements are suspended. The braking element is a flapper valve which opens to permit water flow in an upward direction only and provide a damping amplitude action only for water attempting to flow in a downward direction relative to the damping plate.
The above-disclosed prior art devices do not provide effective reduction of high amplitude waves and are not particularly suitable for use at deep ocean locations. For example, experiments have shown that the devices located within the air-sea interface suffer greatly from damage when exposed to high seas.
The available means to mitigate the large wave effect on various man-made structures, such as drilling platforms, and to protect such structures from the damaging effect, of the waves by building floating platforms which rise and fall a small percentage of the wave height (semi-submersible) is difficult and expensive. The present invention provides a device which overcomes disadvantages of the prior art.